1. 12V is low... it means twice the current is needed to deliver a certain amount of power as compared to 24V. 24V cannot shock you, whereas 48V can. So I suggest 24V

2. The number of winds of coils per phase, the strength of the magnets, the air gap and the speed of rotation all affect the voltage. Yes, it complicated. Maybe build a prototype and see what voltage you get. If the RPM is slow, you need to increase the magnet strenght and/or the number of winds.

3. The question does not make sense. Volts per RPM is a sensible measure. Watts per square meter of blade also makes sense. Watts is power, RPM is speed - to confuse the two is common. A huge turbine turning slowly will produce far more watts (i.e. power) than a tiny one spinning very fast.